1. Field Of The Invention
This invention relates to a developing method and a developer composition and particularly to developing methods and developer compositions for use in electrophotography.
2. Discussion Of The Prior Art
Electrostatic formation of an image on the surface of a photoconductive material and development of the image are known.
For example, a fundamental xerographic process disclosed in U.S. Pat. No. 2,297,691 to C. F. Carlson comprises applying a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light image to thereby cause imagewise dissipation of the electrostatic charge on the layer and thus form an electrostatic latent image, depositing a very finely divided electroscopic material called toner, onto the obtained electrostatic latent image thereby obtaining a toner image corresponding to the latent image, transferring the toner image onto a surface of an image support material such as paper and permanently fixing the transferred image to the image support material by heating means, for example. In the above process, the formation of the electrostatic latent image can be alternatively effected by direct image formation by imagewise electrostatic charging the photoconductive insulating layer. Also, the transfer of the toner image can be dispensed with by directly fixing the image on the photoconductive insulating layer. Furthermore, the thermal fixing can be replaced by other appropriate fixing methods such as utilizing a solvent or a covering.
With respect to the application of the electroscopic powder to the electrostatic latent image, various processes have been proposed, one of which is the so-called cascade developing process disclosed in U.S. Pat. No. 2,618,552 to E. N. Wise. In this process, a developing material consisting of a finely divided toner powder and another powder (called the carrier) of relatively large particle size, the surface of which is electrostatically coated, is conveyed in a conveyor and is cascaded down along a surface bearing the electrostatic latent image. The carrier powder is selected to electrostatically charge the toner powder to a desirable polarity by means of friction charging. As the developing material is cascaded over the electrostatic latent image, the toner powder is attracted electrostatically to the charged areas of the electrostatic latent image and remains fixed thereon while no such adhesion takes place on the uncharged or background areas of the image. A major portion of the toner deposited on the background areas is removed by carrier powder falling thereover because the electrostatic attractive force between the toner powder and carrier powder is stronger than that between the toner powder and the background areas from which electrostatic charge has been dissipated. The carrier powder and excessive toner powder are recycled for repeated use in the process. The above-mentioned process is particularly preferable for developing line images.
Another example of a process for developing an electrostatic latent image is the magnetic brush process disclosed in U.S. Pat. No. 2,874,063. In this process, a developing material containing toner powder and magnetic carrier powder is supported by a magnet and arranged in the shape of a brush which is brought into contact with a surface bearing the electrostatic latent image whereby the toner powder is attracted, by means of an electrostatic attractive force, to the latent image from the magnetic brush.
Another example of a process for developing an electrostatic latent image is the so-called powder cloud developing process disclosed in U.S. Pat. No. 2,221,776 to C. F. Carlson in which developing material consisting of a charged toner powder floating in a gaseous fluid is flowed in the vicinity of a surface bearing the electrostatic latent image whereby the toner is attracted, by means of an electrostatic attractive force, to the latent image from the fluid. This process is particularly preferable for developing an image with continuous tone.
Another example of a process for developing an electrostatic latent image is the liquid developing process disclosed in U.S. Pat. No. 2,899,355 in which a developing material consisting of charged toner powder dispersed in a liquid is flowed over a surface bearing the electrostatic latent image whereby the toner is attracted by means of an electrostatic attractive force to the latent image from the liquid.
Another example of a process for developing an electrostatic latent image is the C-shell developing process disclosed in U.S. Pat. No. 3,503,776. In this process, the toner powder and carrier powder are maintained in continued contact with the drum surface. This process is characterized by low cost and is effective for removing unnecessary toner powder deposited on the background areas.
Another example of a process for developing an electrostatic latent image is the fur brush developing process disclosed in U.S. Pat. No. 2,902,974 in which a fur brush of, for example, cylindrical shape which is dusted with toner powder is rotated in contact with an electrostatic latent image to develop the latent image.
Furthermore, another example of a process for developing an electrostatic latent image is the touch-down developing process disclosed in U.S. Pat. No. 3,166,432 which is employed in certain areas.
Althrough the above-mentioned developing processes are commercially utilized at present, the xerographic developing process most widely employed commercially is the cascade developing process, the use of which in a universal business copying machine is described in U.S. Pat. No. 3,099,943. The cascade development is generally achieved in commercial devices by cascading a developing mixture down the surface of a drum having a horizontal axis and bearing an electrostatic latent. The developing mixture or developing material is conveyed by means of an endless belt conveyor from a trough or a container to a position above the drum, from which the developing material is cascaded down along the surface of the drum and returned to the container after development of the electrostatic latent image. A small amount of toner is synchronously added to the developing mixture in order to replenish the consumption of the toner by development. The toner image thus formed on the drum is ordinarily transferred onto a sheet of paper and then fixed thereon, for example, by fusion with an appropriate device. After the transfer, the surface of the drum is cleaned for repeated use. This developing process is repeated for each reproduction produced in the copying machine, ordinarily several ten thousand times within the effective lifetime of the developing material.
As is obvious from the foregoing explanation, the toner powder is subjected to vigorous mechanical abrasion in any developing process, which eventually leads to pulverizing of the toner powder or smearing of the carrier powder by adhesion of the toner thereon. This effect can be lessened when the toner powder contains a tenacious resin component of high molecular weight capable of withstanding shearing and impact force. Unfortunately, however, most tenacious high-molecular resins are not suitable for use in high-speed automatic copying machines, principally because of the difficulty in rapid fusion of the toner image in the heating step thereof.
On the other hand, the use of a larger heater with an elevated capacity for rapid fusion of toner with a high melting point makes it difficult to prevent the support paper from scorching and to dissipate efficiently the heat generated by the heater. Occasionally the support paper burns after having passed the heater. Thus, the prevention of burning or scorching of the paper generally necessitates additional equipment such as an expensive and complicated cooling device in order to dissipate the large amount of heat generated by the heater. Also, incomplete removal of the generated heat not only gives an unpleasant feeling to the operator of the machine but also damages mechanical components susceptile to heat. Furthermore, the relatively high running cost of such heater and cooling equipment as well as the considerable space occupied thereby frequently cannot be justified by higher machine speed, which results from the use of the heater and cooling equipment.
The above-mentioned drawbacks can be resolved to a certain extent by the use of a heat roll fuser, which fixes the toner image by pressing it while heating it. This fuser provides a satisfactorily fixed image even at a lower temperature of the heating element than that obtainable with radiant heat. Thus, an increase of operating speed is effected to a certain extend, but the above-mentioned problems are still unavoidable if the machine speed has to be further increased.
On the other hand, though the use of a resin of lower molecular weight which can be easily fused and fixed at a relatively low temperature seems to resolve the fixing problems, the use of a heater with increased heating capacity or of a heat roll fuser is still indispensable for a significant increase of operating speed. A heat roll fuser is preferred since the use of a heater of elevated heating capacity will naturally encounter the above-mentioned problems. In this case, the toner consisting of a resin of low molecular weight adheres, at the fusion thereof, not only to the paper but also to the heat roll, leading eventually to severe staining of the fuser lubricator which is a cloth provided to clean the heat roll and finally to staining of the paper itself. Furthermore, most resins of low molecular weight tend to decompose when fused in a high-speed copying apparatus. Further, they tend to form a sticky image which can be offset from one paper sheet to an adjacent sheet. Furthermore, such resins of low molecular weight tend to form a thick film over the photoconductive material which is repeatedly used, and this film deteriorates the toner images thus increasing the down time of the apparatus. Furthermore, it is extremely difficult or rather nearly impossible to pulverize such resins of low molecular weight in a conventional grinding apparatus.
The toner material must be electrostatically charged with a proper polarity when brought into friction contact with the surface of the carrier material in a cascade or touch-down developing device. The charge characteristics by friction and flow of most toner materials are affected by humidity in the air. For example, certain toners show fluctuation in the frictional electricity thereof in accordance with the relative humidity and thus are not particularly usable in a xerographic apparatus. Consequently, a toner with a constant charging characteristic by friction is particularly desirable for automated apparatus.
Another factor affecting the stability of charging characteristics of toner is the tendency thereof to collide with the surface of the carrier, which is observed in certain toner materials. In the course of multiple usage of the developing material in an automatic developing machine, the numerous collisions taking place between the carrier and toner in the device cause the toner powder sticking on the carrier surface to form a film deposit on the surface. Such a gradually increasing deposition of the toner alters the charging characteristics of the carrier, reducing the ability thereof to hold toner powder thereon and eventually deteriorating the image quality of the obtained copy.
The abrasive nature of most carrier and toner materials accelerates other deterioration because of the frictional contact between the toner, carrier and the surface of a xerographic layer, which requires frequent replacement of the xerographic layer which is costly and time consuming.
Furthermore, images obtained by a xerographic process not only have to have a satisfactorily high line image contrast but also must be capable of reproducing a solid black area. It is known, however, that an improvement in either line image contrast or solid area reproduction will generally result in deterioration of the other. More specifically, the application of a larger amount of toner onto an electrostatic latent image in order to increase the image density will generally give rise to undesirable toner deposition on also the background areas.